1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which is provided with a VVT (variable valve timing drive) mechanism, and more specifically, to an estimation device for a cylinder intake air amount in an internal combustion engine, which serves for calculating an amount of intake air sucked in a cylinder with a high degree of accuracy.
2. Description of the Related Art
In general, in order to control an engine in a suitable manner, it is important to calculate an amount of air to be sucked into a cylinder (hereinafter also referred to as a cylinder intake air amount) with a high degree of accuracy, and to carry out fuel control and ignition timing control according to the amount of air which has been sucked into the cylinder.
In order to obtain a cylinder intake air amount, there are generally applied two kinds of methods including an AFS method of measuring the cylinder intake air amount by the use of an air flow sensor (AFS: Air Flow Sensor) which is arranged in an intake pipe at a location upstream of a throttle valve, and an S/D method (Speed Density method) of calculating by estimation the cylinder intake air amount from an intake manifold pressure and an engine rotational speed by using a pressure sensor (hereinafter referred to as an “intake manifold pressure sensor”) which is arranged in an intake manifold system (a surge tank and an intake manifold) downstream of the throttle valve in the intake pipe, and an engine rotation sensor.
In addition, there have also been known a technique which serves to switch between the individual methods according to an operating state of an internal combustion engine by using the above-mentioned sensors in combination with each other, and a technique which serves to measure an intake manifold pressure, even in the case of an AFS method.
In recent years, for the purpose of further reducing fuel consumption as well as further increasing output power, there is made popular an adoption of a VVT (Variable Valve Timing) mechanism (hereinafter referred to as an “intake VVT”) which serves to make variable the valve opening and closing timing of each intake valve. Moreover, VVT mechanisms are also becoming increasingly adopted for exhaust valves, too, in addition to intake valves (hereinafter referred to as an intake and exhaust VVT system).
In an engine provided with such an intake and exhaust VVT system, however, an amount of intake air sucked into a cylinder from an intake manifold changes greatly depending on the valve opening and closing timing of the intake and exhaust valves, as a result of which if the influence of the valve opening and closing timing is not taken into consideration, in particular in the S/D method, the calculation accuracy of the amount of intake air sucked into the cylinder will decrease to a large extent in all the operation regions including a steady state operation region and a transient operation region.
In addition, in cases where the valve timing is caused to change, a response delay will occur, so that at the time of transient operation, valve timing does not match that which has been set at the time of steady state operation, thus resulting in a cause that will reduce the calculation accuracy of the amount of air to a substantial extent.
In the past, as an estimation method for a cylinder intake air amount in the S/D method, there has been known a method of calculating a cylinder intake air amount from an intake manifold pressure, a volumetric efficiency, a cylinder volume, and a temperature, by assuming as a premise that engine parameters such as valve timing, etc., do not change (for example, refer to a first patent document).
In the method of the first patent document, in cases where a variable valve is applied to the S/D method, it can be considered that a volumetric efficiency in the steady state operation, in which valve timing is in coincidence with one in a control map of valve timing, is set to a map value. In this case, however, there will be no problem at the time of steady state operation, but at the time of transient operation, the calculation accuracy of the amount of air will reduce to a substantial extent.
Accordingly, in order to suppress the reduction in the calculation accuracy of the amount of air at the time of transient operation, it is also considered that many maps for volumetric efficiency have been set according to valve timing, but in the case of applying such a scheme to an intake and exhaust VVT system, it is necessary to set maps of volumetric efficiency according to valve timing of each of an intake VVT mechanism and an exhaust VVT mechanism. As a result, a large number of man hours are required in adaptation and data setting, and moreover, the capacity of memory required for a microcomputer in an ECU becomes huge.
For example, according to the method of the first patent document, as for the number of maps for volumetric efficiency corresponding values (indexes each of which indicates an amount of air coming into a cylinder from an intake manifold), in cases where the operating range of a VVT mechanism is represented by six representative points, with each region between adjacent points (hereinafter referred to as an interpoint region) being interpolated, six volumetric efficiency corresponding value maps are required for a system configuration using only an intake VVT mechanism, and a total number (6×6=36) of volumetric efficiency corresponding value maps will be required for a system configuration of an intake and exhaust VVT system.
That is, in an engine having VVT mechanisms, in cases where an S/D method of estimating a cylinder intake air amount from an intake manifold pressure and an engine rotational speed is applied, it is necessary to adapt the volumetric efficiency corresponding value according to the actual valve timing of each VVT mechanism, and hence the number of storage maps becomes huge.